The term “battery” originally meant a plurality of electrochemical cells connected in series in a housing. However, even single electrochemical cells are nowadays frequently referred to as a battery. During discharge of an electrochemical cell, an energy-supplying chemical reaction made up of two electrically coupled, but spatially separated part reactions takes place. A part reaction which takes place at a relatively low redox potential proceeds at the negative electrode and a part reaction takes place at a relatively high redox potential at the positive electrode. During discharge, electrons are liberated by an oxidation process at the negative electrode, resulting in an electron current which flows via an external load to the positive electrode which takes up a corresponding quantity of electrons. Thus, a reduction process takes place at the positive electrode. At the same time, an ion current corresponding to the electrode reaction flows within the cell. This ion current is ensured by an ionically conductive electrolyte. In secondary cells and batteries, this discharging reaction is reversible, i.e. it is possible to reverse transformation of chemical energy into electric energy which occurred during discharge. Where the terms “anode” and “cathode” are used in this context, the electrodes are generally named according to their function during discharging. The negative electrode in such cells is thus the anode, and the positive electrode is the cathode.
Among secondary cells and batteries, comparatively high energy densities are achieved by cells and batteries based on lithium ions. These generally have composite electrodes which comprise not only electrochemically active components, but also electrochemically inactive components. Possible electrochemically active components (often also referred to as active materials) for cells and batteries based on lithium ions are essentially all materials which can take up lithium ions and release them again. In this context, particles based on carbon, in particular, e.g. graphitic carbon, or nongraphitic carbon materials capable of intercalation of lithium are known for the negative electrode. Furthermore, it is also possible to use metallic and semimetallic materials which can be alloyed with lithium. Thus, for example, the elements tin, antimony and silicon are able to form intermetallic phases with lithium. For the positive electrode, the active materials used industrially at the present point in time comprise, in particular, lithium-cobalt oxide (LiCoO2), LiMn2O4 spinel, lithium-iron phosphate (LiFePO4) and derivatives such as LiNi1/3Mn1/3Co1/3O2 or LiMnPO4. All electrochemically active materials are generally present in particle form in the electrodes.
As electrochemically inactive components, mention may be made first and foremost of electrode binders and power outlet leads. Electrons are supplied to or conducted away from the electrodes by power outlet leads. Electrode binders ensure the mechanical stability of the electrodes and contacting of the particles of electrochemically active material with one another and with the power outlet lead. Conductivity-improving additives, which can likewise be subsumed under the collective term “electrochemically inactive components”, can likewise contribute to an improved electric connection between the electrochemically active particles and the power outlet lead. All electrochemically inactive components should be electrochemically stable at least in the potential range of the respective electrode and have a chemically inert character in the presence of customary electrolyte solutions. Customary electrolyte solutions are, for example, solutions of lithium salts such as lithium hexafluorophosphate in organic solvents such as ethers and esters of carbonic acid.
An electrode active material containing nanosize silicon particles is known from WO 2010/014966 A1. The particles are embedded in a polymer electrolyte, optionally, together with carbon particles. The polymer electrolyte is able to equalize volume changes of the silicon and optionally carbon particles during charging and discharging operations.
US 2006/0035149 A1 discloses an electrode active material which can have carbon fibers in addition to silicon-carbon composite particles.
US 2005/0136330 A1 and US 2009/0252864 A1 disclose electrode active materials comprising silicon-carbon composite particles for lithium ion batteries. The composite particles are produced by coating silicon particles with a coating material selected from the group consisting of petroleum, tar, phenolic resins, sugars, polyacrylonitrile and lignin, followed by pyrolysis of the decomposition material.
An important factor for the performance of secondary lithium ion cells is the fact that even during the first charging/discharging cycle of such cells (known as activation), a covering layer which generally consists of electrolyte decomposition products and oxidized lithium is formed on the surface of the electrochemically active materials in the anode. The covering layer is referred to as “solid electrolyte interface” (SEI). The SEI is in the ideal case permeable only for the extremely small lithium ions and prevents further direct contact of the electrolyte solution with the electrochemically active material in the anode. In this respect, formation of the SEI has positive effects. However, a disadvantage is that mobile lithium is lost in the formation of the SEI and at the same time the internal resistance of the cell increases.
It could therefore be helpful to provide a novel, alternative electrode active material which makes it possible to construct batteries having a relatively high energy density, but which at the same time has fewer disadvantages than the abovementioned known active materials.